1. Field of the Invention
This invention pertains to a pneumatic impact mechanism in which a drive piston and a percussion piston move axially within an impact mechanism housing.
2. Description of the Related Art
These types of impact mechanisms are commonly used in hammer drills and sledgehammers and in practice two types of impact mechanisms have proven useful, among others. The first type consists of an impact mechanism with a hollow drive piston moved inside the impact mechanism housing, in the cavity of which the percussion piston is moved. The other type consists of an impact mechanism with a hollow percussion piston moved inside the impact mechanism housing, in the cavity of which the drive piston is moved. The commonality of both impact mechanism types is that the drive piston is driven by a crankshaft, for example, and that a pneumatic spring is created between the drive is piston and the percussion piston that transfers the drive motion of the drive piston onto the percussion piston and forces it in a direction of impact where it finally meets a tool, for example a chisel, transferring its impact energy onto it. Afterward, the percussion piston rebounds and another impact motion starts, supported by the drive piston.
The advantages of the impact mechanism types described are in their minimal requirements with regard to sealing of the separating joints so that robust steel-steel glide pairs can be used in the high pressure range without using additional sealing elements. Moreover, the impact mechanisms exhibit good startup behavior at low temperatures.
Nevertheless, under certain operating conditions, the problem arises in that, after an impact, the return motion of the percussion piston is not sufficient to make a forceful impact, despite the recoil impulse and the suction effect of the drive piston.
The objective of the invention is to provide a pneumatic impact mechanism in which the recoil behavior of the percussion piston is improved.
According to the invention, the objective is met by means of a pneumatic impact mechanism with a impact mechanism housing, a drive piston that is driven axially back and forth, an axially moving percussion piston located in front of a drive surface of the drive piston coaxial to the drive piston as seen in an impact direction, a first chamber in front of the drive surface of the drive piston and located behind a rear surface of the percussion piston, a second chamber located behind the drive surface of the drive piston and with a third chamber located in front of the rear surface of the percussion piston, wherein the second chamber and the third chamber can be made to communicate with one another by means of a connection channel.
The design of the pneumatic impact mechanism enables the drive piston, when it makes a forward motion, to transfer its energy to the percussion piston through a pneumatic spring created in the first chamber, thus transferring its energy indirectly onto the tool. When the drive piston makes a backward motion, air pressure forms in the second chamber located behind the drive piston. This air pressure is passed through the connection channel to the third chamber in front of the percussion piston. In this way, when the drive piston is moved backward, the return motion of the percussion piston is supported independent of its recoil after the impact and independent of the suction effect of the drive piston transferred by the first chamber. A reliable return motion of the percussion piston is the result even under difficult operating conditions, so that when the drive piston repeats its forward motion, another forceful impact can be made.
The communication between the second chamber and the third chamber enables the pressure change in the second chamber caused by the motion of the drive piston to change the pressure in the third chamber by means of the connection channel.
In a first preferred embodiment form of the invention, the drive piston is moved inside the impact mechanism housing whereas the percussion piston is moved inside a cavity formed at one end of the drive piston.
Alternatively, in another very advantageous embodiment form, the percussion piston is moved inside the impact mechanism housing whereas the drive piston is moved inside a cavity formed in an end face. The solution according to the invention is suitable for both of the pneumatic impact mechanisms mentioned.
In a preferred development of the invention, the second chamber is located between a rear surface of the drive piston and a rear tubular base fastened to the impact mechanism housing, whereas the third chamber is located between a forward surface of the percussion piston and a rear tubular base fastened to the impact mechanism housing. This enables additional chambers, as compared to the state of the technology, to be created behind the drive piston and in front of the percussion piston without expensive additional design measures.
In the process, the drive piston is designed such that it has a piston head that constitutes both the drive surface and the rear surface, a bracket with which to fasten to a drive unit and a center member that connects the piston head to the bracket. This design makes it possible to locate the tubular base between the piston head and the bracket, which creates the second chamber in a simple manner.
In another advantageous configuration of the invention, an idle channel is provided that has at least one idle opening provided in a wall of the drive piston and which penetrates a wall of the impact mechanism housing. The idle channel is connected either to the connection channel or the outside. Through the idle channel, it is possible to short circuit the first and second chambers so that no pressure relationship can form in the pneumatic impact mechanism that acts on the percussion piston when the pneumatic impact mechanism is at idle.
In an especially advantageous embodiment form, a shifting control slide is provided that can switch between an impact position and an idle position. When it is in the impact position, it creates the connection between the second and third chambers by means of the connection channel while blocking the idle channel. In the idle position, it blocks off the connection channel and opens the idle channel, thus precisely bringing about the transfer between idle and impact positions. It is advantageous here to carry out the axial shifting of the control slide by coupling the control slide to the tool or to the die located between the percussion piston and the tool. When switching to idle, the tool or the die slides somewhat forward out of the housing when lifted away from the rock, with the control slide also following this motion.